Two lenses with the same focal lenght and speed but with different purpose since the Pentax was made during 35mm SLR era and the Sigma is a brand new design optimized for APS-C MLC (and M 4/3 cameras). I bought my Pentax FA20 around 1995-1997 for about 850$ CDN (Canadian) and the Sigma just cost 240$ (CDN with shipping and taxes). The most evident physical difference is the diameter of the front of the lens, the Pentax takes 67mm filters and the Sigma takes 46mm filters ! The Sigma have 7 aperture blades and the Pentax have 6, they are more rounded on the Sigma so it should produce a better Bokeh.
The build quality is better on the Pentax , which is not a surprise, the Sigma comes with a lens hood and i had to buy it separately for my Pentax…. now, the Pentax lenses always comes with a lens hood. The minimum focusing distance of the Pentax is 0.25 meter and for the Sigma it’s 0.20 meter.
I will test both of them on my Sony NEX-3 to see how they compares to each others.
Here is a photo taken in the field with both lenses on my NEX-3 from the same place, you can see that the Sigma’s 1mm advantage is evident here, the plant is not cut in the upper part of the frame.:
– Pentax FA20mm/2.8:
– Sigma 19mm:
Now, here is my test set-up inside my house for sharpness and CA. My tripod was set to it’s lowest position and at 0.7 meter from “Fraisinette” . I use a bubble level to be sure that the camera was leveled side to side and that it was not tilted down in the front.
All the photos were taken in RAW at ISO 200 with the 2 second delay, manual focus was used with the help of the focus peaking. The photos were developed in Lightroom where i turned the sharpening to zero and nothing else was done in PP after that, only resized and saved them in Jpeg for the web.
Now the results:
Sigma at f/2.8.
Pentax at f/2.8.
We can see that the Sigma is sharper and have better contrast than the Pentax at f/2.8. After that the Pentax is always sharper than the Sigma all the way up to f/11, from f/16 diffraction kick in and they are very similar in performance.
Sigma at f/8.
Pentax at f/8.
The Sigma 19mm is knowned for it’s CA, but what about the Pentax ?
Well the Sigma showed CA at all apertures while the Pentax is free of CA from f/5.6. The CA is not that visible here but easier to see while zooming in Photoshop. You can see it along the white dress on the right side of the photo.
Sigma CA at f/2.8.
Sigma CA at f/11.
Pentax CA at f/2.8.
If you need to shoot often at f/2.8 then the Sigma 19mm is the one to buy, Pentax lenses are not always the sharpest W/O but sharpens nicely when stopped down and the FA20 is no exception here. The corners in the Pentax shots are better since this is a FF lens but the Sigma is still very good when stopped down.
They are both very good lenses, the Pentax is a tad sharper and shows more details when viewed at 100% but in normal use and when printed you may have difficulties setting them apart. The Sigma have AF on an NEX camera and is smaller since the Pentax needs an adapter. So for the price the Sigma is a no brainer.
The Sigma is a modern lens and the Pentax is a legacy lens, i will continue using my Pentax FA20mm on my K-01 and my Sigma 19mm on my NEX-3.
Some shots taken with both lenses:
Sigma 19mm at f/5.6.
Sigma at f/11.
Pentax FA20mm at f/13 with Polarizer.